In checking my river charts, I find only one sunken steamboat which might possibly be in the present river channel near the location you've described. That would be the CORVETTE, which sank just above what is now the 153 mile marker in 1842. I don't have much information as to size on the CORVETTE, except that she was about 180 feet long. She was wrecked by a snag in 1842. I don't have the exact date.
Two others, the E. A. OGDEN (1859) and the WENONA (1855) both sank below the 153 mi. marker (actually closer to the 152...and both are shown on my charts as being under solid ground now rather than in the current channel. The E. A. OGDEN, which was wrecked by a snag, was actuallly salvaged, and parts from the boat were sold at auction in St. Louis in November 1859. I'm in the process of trying to research the possibility that Waltus Watkins (who built Watkin's Woolen Mill near Kearney, Missouri) might have purchased the boiler from this boat to use in his mill.
It is possible that this might be a hull from a later boat...it would take some serious underwater archaeology to determine just exactly what you've found. Please feel free to e-mail me directly at: firstname.lastname@example.org and try sending the picture.
This is definitely an interesting mystery...